Field of the Invention
The invention relates to a method for removing tin from the surface and/or from a region close to the surface of the component part. Furthermore, the invention relates to an application of the method to a component part, in particular a turbine blade.
European Patent Application 0 631 832 A1 describes the fabrication of a component part by unidirectional solidification of a metal melt. That involves filling a mold with an alloy melt which forms the component part, wherein the mold is maintained in a heating chamber at a temperature above the melting temperature of the alloy. The mold containing the molten alloy is introduced continuously into a bath having a cooling fluid, with unidirectional solidification thus being induced. In order to avoid heat currents and to establish a high temperature gradient, the cooling fluid is covered with a thermally insulating layer that is made of free-flowing material, through which the mold is lowered into the cooling fluid.
A possible cooling fluid is molten tin having a temperature of about 260.degree. C., as described in German Published, Non-Prosecuted Patent Application DE 22 42 111 A1. If the mold is cooled through the use of a liquid metal (liquid metal cooling or LMC), there is the possibility that the liquid metal, for example liquid tin, may come into direct contact with the solidified alloy through cracks in the mold. In that case, the tin may adhere to the surface (tin accretion) or form a compound with the alloy in a region close to the surface (tin enrichment). In the course of a heat treatment of the cast component part, which may follow the casting process, there is the possibility of more tin diffusing into the component part. There is a consequent risk of the mechanical properties and the corrosion characteristic of the component part being impaired.
German Published, Non-Prosecuted Patent Application DE 36 04 792 A1 describes a method for the fabrication of tin-free articles by deep-drawing or ironing tin-plated sheet iron. A tin coating on a latten, for example for making tin cans, is removed through the use of a strong alkali such as sodium hydroxide.